|
FLASH the free electron laser at Hamburg as a part of the Deutsches Elektronen Synchroton DESY is the first Free Electron Laser (FEL) user facility for VUV and soft X-ray coherent light experiments. The SASE (Self Amplification by Stimulated Emission) process generates ultra short coherent radiation pulses in the femto seconds time range with peak powers in the GW level. Several experiments need reliable means to control the FEL intensity over many orders of magnitude without changing the beam characteristics delivered from the FEL. Since a reduction of the FEL intensity by variation of machine parameters is not appropriate, a windowless gas-filled cell in combination with differential pumping units is used for attenuating the FEL radiation. This attenuator is placed in the ultra high vacuum system of the photon beamline in front of the experimental hall. The total length of the gas cell is approximately 15 m with a minimal aperture of 20mm for the differential pumping units. The maximum gas pressure in the attenuator is about 0.1 mbar. This is handled by differential pumping, which conserves the ultra high vacuum conditions of the beamline. The attenuation range of Nitrogen covers at least 5 orders of magnitude in the spectral range of 19 to 120 nm due to its large absorption cross section. The technical layout of the pumping systems using SplitFlow® turbomolcular pumping units combined with inline ion getter pumps is described in detail. Experimental experiences with the gas attenuator are presented. |